There are all kinds of things in the ocean. The ocean floor is loaded with all sorts of stuff like detritus, mud, and the like. A closed system is a completely different ball game. You do not want to disturb a sand bed that is full of detritus because it will overload your small water volume with nutrients and cause you problems.
The sand we add to our tanks is void of mud and such and will not release anything. It's basically comprised of whatever it's made of- calcium, or silica. Nothing will come out of it.
Once it's been in the tank for a while, it will start to build up nutrients. Nutrients that you don't want in your water column.